Swiss Transfer Ticket and birth date of shild

i am attempting to purchase transfer tickets for our upcoming trip from Geneva to Interlacken my sons DOB is 030/05/2002 however it will not accept this DOB for free travel with accompanying adult . Am I misunderstanding somethings Thanks in advance

The site Alpenrose suggested is the right one. They do ask for the birth date after selecting the "Online Ticket" option to print at home. I had no trouble selecting May 30 2002. Did it work for you now on this website? Other websites selling the Swiss Transfer Ticket are listed here: myswissalps.com/ swisstransferticket/ price.